1. Field of the Invention
This invention is directed to a catheter intended for implantation within a living body for long term operation. The catheter is intended for use, for example, with the implantable infusion pump or artificial gland described in U.S. Pat. No. 3,731,681, or with other implanted injection ports for drug delivery to internal organs or sites not accessible by direct needle puncture.
2. The Prior Art
The major problems with long term catheterization of a blood vessel are infection at the skin puncture point and clotting in the catheter lumen. Only in cases where the flow rate of infusate is very high, or if an anti-coagulant is being infused, does the catheter tip remain open for a long term. In applications such as insulin infusion, the flow rates are very low, and back diffusion of blood components can result in clots or plugs forming inside the lumen of the catheter. This limits the useful lifetime of the indwelling catheter in many instances to only three to four months.
In the case of use of an implanted infusion pump such as that of aforesaid U.S. Pat. No. 3,731,681, a further complication exists in that the catheter does not always have a forward flow. For example, when the pump is being filled, it is possible to momentarily generate suction which pulls blood retrograde back into the catheter and this blood inevitably clots. Also, changes in blood pressure may cause a fluctuating pressure at the tip of the catheter and can induce backward flow up the catheter. The cyclic pressure pumps fluid along the wall where the velocity is low. The catheters are always designed with a small exit hole to keep the velocity of the infusate as high as possible. However, this also has danger in that the cross section is very small and can be easily plugged. The plug can come from material that diffuses upstream from the blood or from small particles which come downstream out of the pump itself. If the hole is made very small to obtain high velocity, it is difficult to keep it from plugging due to particulates in the infusate. The ideal catheter is one in which it is not necessary to maintain a fixed flow rate to keep it open. Therefore, the catheter flow rate may be allowed to go to zero for long periods of time. The catheter should also be able to pass particles that come downstream with the infusate without plugging the tip. In order to resist back diffusion, the exit hole should be as small as possible to maintain a critically high exit velocity. The catheter of the present invention incorporates these desirable features and overcomes the shortcomings of prior art devices.